The invention relates to a computer sewing machine having two pulse motors, one for controlling the needle swinging amplitude and the other for controlling the fabric feeding amount under a predetermined program of microcomputer. More particularly, the invention relates to a computer sewing machine of Janome, in which a sensor is provided to detect an accident in operation of the sewing machine. Namely, if the drive shaft of the sewing machine is prevented from rotation for a predetermined time upon energization of the machine drive motor due to an accident such as the jamming of loop taker, the sensor detects the abnormal condition (failure in rotation of the drive shaft), and produces an electric signal. Then the microcomputer is operated with the electric signal to deenergize the machine drive motor even if the machine operator operates the controller switch of the motor to avoid the overheat of the machine drive motor. A failure in rotation of the drive shaft is caused not only by an accident but also by declutching the belt wheel from the drive shaft just when the bobbin thread winding operation is carried out. As can be seen, the bobbin thread is wound with the belt wheel which is rotated by energization of the machine drive motor while the drive shaft is left standstill. It is, however, undesirable if such a microcomputer detects failure of the drive shaft rotation and then deenergizes the machine drive motor, just as in the case of some accident. Therefore it becomes necessary in such an occasion to provide a device to produce a signal indicating that the microcomputer should not operate to deenergize the machine drive motor.
In conventional computer sewing machines using two pulse motors for controlling the needle swinging amplitude and the fabric feed amount the pulse motors are each provided with a photoelectric sensor 5 and a screening element 4 illustrated on FIG. 1 which shows together with FIG. 2 a prior art arrangement. Sensor 5 and screening element 4 are cooperated so as to determine the initial position of each pulse motor when the power source is applied to the sewing machine in the manner as described in detail in Janome U.S. Pat. No. 4,271,773. Additionally another sensor (microswitch 11) is used in relation with the bobbin thread winding device, especially in Janome computer sewing machine. The sensor 11 is operated in association with the actuating member 6 which is manually operated to bring the wheel 46 into engagement with the belt wheel of the sewing machine for winding the bobbin thread and simultaneously to cause the declutch pin 9 to declutch the belt wheel from the drive shaft of the sewing machine. The sensor 11 is operated to produce an electric signal indicating that the microcomputer should not deenergize the machine drive motor if the drive shaft is not rotated when the controller switch of the machine drive motor is operated to drive the belt wheel for winding the bobbin thread.
The conventional Janome computer sewing machine has three sensors for the two pulse motors and for the bobbin thread winding device, respectively.